Promotion Effect of the X-Zeolite Host on Encapsulated Platinum Clusters for Selective Hydrogenation of Phenylacetylene to Styrene

Preparation of Metal-Supported Nanostructured Zeolite Catalysts and their Applications in the Upgrading of Biomass-Derived Furans: Advances and Prospects

The development of platform chemicals derived from biomass, in particular, 5-hydroxymethylfurfural (5-HMF) and furfural (FUR), is of crucial importance in biorefinery. Over the past decades, metal-supported nanostructured zeolites, in particular, metal-supported hierarchically porous zeolites or metal-encapsulated zeolites, have been extensively elaborated because of their multiple functionalities and superior properties, for example, shape-selectivity, (hydro)thermal stability, tunable acidity and basicity, redox properties, improved diffusion, and intimacy of multiple active sites. In this review, the effects of such properties of metal-supported nanostructured zeolites on the enhanced catalytic performances in furanic compound upgrading are discussed. In addition, the recent rational design of metal-supported nanostructured zeolites is exemplified. Consequently, the ongoing challenges for further developing metal-supported nanostructured zeolites-based catalysts and their applications in HMF and FUR upgrading are identified.

Encapsulation of Pd Single-Atom Sites in Zeolite for Highly Efficient Semihydrogenation of Alkynes

Palladium (Pd)-based single-atom catalysts (SACs) have shown outstanding selectivity for semihydrogenation of alkynes, but most Pd single sites coordinated with highly electronegative atoms (such as N, O, and S) of supports will result in a decrease in the electron density of Pd sites, thereby weakening the adsorption of reactants and reducing catalytic performance. Constructing a rich outer-shell electron environment of Pd single-atom sites by changing the coordination structure offers a novel opportunity to enhance the catalytic efficiency with excellent alkene selectivity. Therefore, in this work, we first propose the in situ preparation of isolated Pd sites encapsulated within Al/Si-rich ZSM-5 structure using the one-pot seed-assisted growth method. Pd1@ZSM-5 features Pd-O-Al/Si bonds, which can boost the domination of d-electron near the Fermi level, thereby promoting the adsorption of substrates on Pd sites and reducing the energy barrier for the semihydrogenation of alkynes. In semihydrogenation of phenylacetylene, Pd1@ZSM-5 catalyst performs the highest turnover frequency (TOF) value of 33582 molC═C/molPd/h with 96% selectivity of styrene among the reported heterogeneous catalysts and nearly 17-fold higher than that of the commercial Lindlar catalyst (1992 molC═C/molPd/h). This remarkable catalytic performance can be retained even after 6 cycles of usage. Particularly, the zeolitic confinement structure of Pd1@ZSM-5 enables precise shape-selective catalysis for alkyne reactants with a size less than 4.3 Å.

Reversible Intrapore Redox Cycling of Platinum in Platinum-Ion-Exchanged HZSM-5 Catalysts

Isolated platinum(II) ions anchored at acid sites in the pores of zeolite HZSM-5, initially introduced by aqueous ion exchange, were reduced to form platinum nanoparticles that are stably dispersed with a narrow size distribution (1.3 ± 0.4 nm in average diameter). The nanoparticles were confined in reservoirs within the porous zeolite particles, as shown by electron beam tomography and the shape-selective catalysis of alkene hydrogenation. When the nanoparticles were oxidatively fragmented in dry air at elevated temperature, platinum returned to its initial in-pore atomically dispersed state with a charge of +2, as shown previously by X-ray absorption spectroscopy. The results determine the conditions under which platinum is retained within the pores of HZSM-5 particles during redox cycles that are characteristic of the reductive conditions of catalyst operation and the oxidative conditions of catalyst regeneration.

Enhanced Alkene Selectivity for Transfer Semihydrogenation of Alkynes over Electron-Deficient Pt Nanoparticles Encapsulated in Hollow Silica Nanospheres

In this work, we report that Pt nanoparticles confined in hollow porous silica nanospheres (Pt@HPSNs) function as highly selective catalysts for the transfer hydrogenation of phenylacetylene to styrene with ammonia borane. Relative to the deep hydrogenation of phenylacetylene to ethylbenzene over the supported Pt/SiO2, Pt@HPSNs exhibit above 88% of styrene selectivity at nearly 100% of phenylacetylene conversions, and the high selectivity of Pt@HPSNs can be maintained even at high ammonia borane/phenylacetylene ratios and longer reaction time. The Pt 4f X-ray photoelectron spectrum of Pt@HPSNs shows a remarkable ∼1.5 eV shift to high binding energy, proving the nature of electron deficiency of such encapsulated Pt nanoparticles. Combined with extremely minor transfer hydrogenation of styrene to ethylbenzene when styrene as substrates, the enhanced styrene selectivity of Pt@HPSNs is ascribed to the electron deficiency of encapsulated Pt nanoparticles, which leads to the fast desorption of styrene and thus avoids deep hydrogenation.

Metal Sites in Zeolites: Synthesis, Characterization, and Catalysis

Zeolites with ordered microporous systems, distinct framework topologies, good spatial nanoconfinement effects, and superior (hydro)thermal stability are an ideal scaffold for planting diverse active metal species, including single sites, clusters, and nanoparticles in the framework and framework-associated sites and extra-framework positions, thus affording the metal-in-zeolite catalysts outstanding activity, unique shape selectivity, and enhanced stability and recyclability in the processes of Brønsted acid-, Lewis acid-, and extra-framework metal-catalyzed reactions. Especially, thanks to the advances in zeolite synthesis and characterization techniques in recent years, zeolite-confined extra-framework metal catalysts (denoted as metal@zeolite composites) have experienced rapid development in heterogeneous catalysis, owing to the combination of the merits of both active metal sites and zeolite intrinsic properties. In this review, we will present the recent developments of synthesis strategies for incorporating and tailoring of active metal sites in zeolites and advanced characterization techniques for identification of the location, distribution, and coordination environment of metal species in zeolites. Furthermore, the catalytic applications of metal-in-zeolite catalysts are demonstrated, with an emphasis on the metal@zeolite composites in hydrogenation, dehydrogenation, and oxidation reactions. Finally, we point out the current challenges and future perspectives on precise synthesis, atomic level identification, and practical application of the metal-in-zeolite catalyst system.